In 1924, Nicolas Clement first defined a calorie as a unit of heat. A calorie is then, in simplest terms, a unit of energy. In more specific terms, a calorie is the amount of energy, or heat, that it takes to raise the temperature of one gram of water one degree celsius (1.8 degrees Fahrenheit) at one atmosphere pressure. A common unit of energy used in the physical sciences is a "joule." One calorie is equal to 4.184 joules.

Although we tend to associate calories with discussions of food, they can actually apply to any substance containing energy. As an example, one gallon of gasoline (approximately 4 liters), contains about 31,000,000 calories. That's right, 31 million! In today's world, though, the use of the word "calorie" is considered obsolete when referring to anything but food energy. All other heat measurements, as in the gasoline example used above, are more correctly noted in joules.

As we know, human beings require energy to live. They need energy to breathe, energy to pump blood through their bodies, energy to walk, to dance, to run. Essentially, life is energy, and this energy is acquired from food.

Calorie vs. Kilocalorie
What most people don't realize is that the calories we refer to when studying a food label aren't truly calories. They are actually kilocalories. A kilocalorie is equal to 1,000 of the calories discussed above. So, for example, if an banana is said to have 80 calories, in actuality it has 80 kilocalories, or 80,000 'true' calories. At one time, food calories were denoted with a capital 'C' to denote kilocalories. This practice is no longer universally followed, however.

A calorie is also a measure of how much energy the body uses when it performs any activity. Like food calories, these calories are also actually kilocalories. So, when you read that you will expend 100 calories by walking around the block, in truth you are expending 100 kilocalories - or 10,000 calories.

For the purposes of the remainder of this article, whenever "calorie" is mentioned, it will be a reference to kilocalorie.

How Are Calories Calculated?
As we learned earlier, the number of calories in any food is measured by how much potential energy is in that food. Scientists have determined that one gram of a carbohydrate contains 4 calories, one gram of a protein contains 4 calories, and one gram of a fat contains 9 calories. As a result, if you know how many proteins, carbohydrates and fats any foodstuff contains, you can calculate how many calories are contained in that food. You can experiment with these calculations by looking at any food label that breaks down these food components.

How Does Your Body Use Calories?
When we eat food, our bodies "burn", or release the energy from, these calories through a system of various metabolic processes. Different food components are broken down into different materials. For example, carbohydrates are broken down into glucose and other sugars. Fats are broken down into fatty acids and glycerol. Proteins are broken down into various amino acids.

Once food is broken down into its basic component molecules, these molecules travel through the bloodstream to the individual cells. Once in the cells, the energy is available for the body to use. If you take in more calories than your body needs to exist, the excess energy will be stored by your body and you will gain weight. If you accumulate and store 3,500 extra calories you will gain one pound of fat, which is how your body stores excess energy. If you burn 3,500 more calories than you eat, your body will convert 3,500 calories of stored energy, and you will lose one pound of fat.

Does Calorie "Type" Matter?
For the sole purpose of weight gain or loss, the simple truth is that it does not matter whether the calories you consume come from carbohydrates, proteins or fats. A calorie is a calorie regardless of its origin. You will always gain weight if you consume more calories than you burn, or lose weight if you burn more calories than you take in.

It should be noted, however, that your body needs certain nutrients in order to perform at its optimum level and for you to remain healthy. It is for this purpose that scientists recommend that you eat or avoid certain foodstuffs.